1. Field of the Invention
The present invention relates to methods for recycling sludge during waste-water treatment, more particularly to a method for recycling sludge during waste-water treatment whereby sludge of high concentration of copper ions will be leached in sulfuric acid of concentration 1-2N, and, through a process of separating liquid and solid, a solution rich in copper (18-39 g/L) and residues will be obtained. The liquid extractive is then processed in a refrigeration crystallization tank to form copper sulfate crystals. Further, the residues produced are added into the coagulation tank as a coagulant in the process of chemical coagulation for wastewater treatment, thereby achieving a goal of zero sludge production.
2. Description of the Prior Art
The development of industries such as printed circuit board manufacture, metal processing, chemical material production, electroplating, and leather processing facilitates our daily life but at the same time produces a variety of wastes that cause impacts on the environment, especially heavy metal sludge. The amount of heavy metal sludge produced annually in Taiwan has exceeded 320,000 metric tons.
The most frequently used treatment of heavy metal waste-water of the prior art is chemical coagulation and precipitation, whose process is shown in FIG. 1. The system that carries out the process includes: a waste-water equalization tank 11 for storing raw waste-water and for adjusting the variable concentration on the waste-water; a coagulation tank 12 for adding a coagulant so as to increase the concentration of metallic ions; a pH adjustment tank 13 for adjusting the pH value of the waste-water so that the water is basic, thereby facilitating the formation of metallic hydroxides; a flocculated tank 14 for adding a polymer flocculating agent for accelerating the precipitation of the metallic hydroxides; a precipitation tank 15 for separating the metallic hydroxides and the upper-layer waste-water; a filtering unit 16, further comprising a water draining tank 160, a sand filter 161, and an active carbon adsorption apparatus 162, for filtering off other impurities and materials that deteriorate water quality within the upper-layer waste-water; a neutralization and effluent tank 17 for adjusting the pH value of processed water so that it can be released; a sludge thicken tank 18 for enriching the precipitated materials on the bottom of the precipitation tank 15 and for driving the upper-layer water back to the waste-water equalization tank 11 so as to get mixed with the raw waste-water therein; and a sludge hydro-extractor 19 for dehydrating the sludge from the sludge thicken tank 18.
The above process processes waste-water into effluent water and cakes of concentrated heavy metal sludge. The effluent water will be released to the environment, and the sludge cakes will be going through other treatment processes or simply put into a terminal storage.
However, the conventional method of chemical coagulation may produce effluent water that satisfies an environmental standard, but it induces a large amount of heavy metal sludge, which is more hazardous to the environment if not processed well. Further, the treatment of the sludge will increase the cost.
Based on the disadvantage of the conventional method of chemical coagulation, the present invention utilizes the method of hydrometallurgy to extract the copper element from the heavy metal sludge. The pernicious heavy metal sludge is leached in sulfuric acid for dissolving the copper therein, which copper is then collected by an enrichment technique, such as the copper sulfate recycling method. Further, the residues produced in a filtering process are used as the coagulant in the process of chemical coagulation for wastewater treatment, replacing the conventional coagulants such as FeCl3, PAC, FeSO4 and therefore achieving a goal of zero sludge production.